Electrical circuit designers often face the problem of needing to implement electrical circuits using as little space as is practical. Three-dimensional circuit implementations help in miniaturizing circuit area. In three-dimensional circuits, interconnections between components are routed in at least two planes, where the planes are spaced apart and insulated from one other.
While significant space savings can be achieved in a three-dimensional circuit, provisions must be made to accommodate the routing of conductors from one plane to another. Typically, conductors are routed from one plane to another by a feedthrough or "via". A conventional via is made by forming a hole or "barrel", through a substrate between opposing sides of the substrate, applying a conductive material to the barrel (i.e. the substrate walls surrounding the interior of the hole), and connecting the conductive material to conductors formed on the opposing substrate sides. Many processes are known for forming holes, including punching, drilling, and etching. Likewise, many processes, including screen printing, plating, evaporation deposition, and sputter deposition, are known for applying a conductive material to barrels and for forming conductors on opposing substrate surfaces.
However, conventional vias often consume an undesirably large amount of space. Regardless of the particular hole metallization process selected, an aspect ratio parameter limits the minimum amount of surface area which is dedicated to a via on the opposing sides of the substrate. The aspect ratio is calculated by dividing the thickness of the substrate, e.g., length of the via's barrel, by the diameter of the via's barrel. As a rule of thumb, a reliable via requires an aspect ratio of 1:1 or less.
Larger via diameters generate lower aspect ratios, but larger via diameters consume greater amounts of substrate surface area. When a substrate includes several vias, and each via is surrounded by a conductive land area to insure contact with surface traces, much of the substrate's overall area is dedicated only to vias. Of course, a low via aspect ratio may be obtained by narrowing the substrate. However, substrates which are too thin are often not acceptable because of a increased risk of breaking.
If a via is attempted with an aspect ratio greater than 1:1, the via often ends up without uniform metallization throughout the via. Corners and the center of the barrel, between the opposing sides of the substrate, often receive inadequate coverage. When non-uniform coverage occurs, opens may form after several temperature cycles, plating processes may not be successful, and etching processes may form voids in the metallization. Moreover, at an aspect ratio of greater than 1:1, inspection becomes virtually impossible because insufficient viewing angles and illumination are present for the center of the barrel. Consequently, the same via geometries which reduce the chances of obtaining reliable vias prevent inspections from detecting whether resulting vias are in fact acceptable.